At about 5pm on 22 April 1915, French and Algerian troops on the Ypres front in Belgium noticed a lull in the German artillery fire that had been targeting their lines. Bracing themselves for an expected infantry advance, they were puzzled instead to observe a greenish-yellow cloud drifting towards them, then lapping over the tops of the trenches.

A Canadian soldier, AT Hunter, who witnessed what was the first use of chlorine gas in war, described a “passive curiosity turned to active torment – a burning sensation in the head, red-hot needles in the lungs, the throat seized as by a strangler. Many fell and died on the spot. The others, gasping, stumbling with faces contorted, hands wildly gesticulating, and uttering hoarse cries of pain, fled madly through the villages and farms and through Ypres itself, carrying panic to the remnants of the civilian population.”

Watching the gas being released from behind the German lines was the “father” of chemical weapons, the Nobel-winning scientist Fritz Haber. Haber, a German who was both Jewish and a fervent nationalist, saw his contribution of gas to the war effort as a way to prove his patriotism. He had already worked on new ways of mass producing high explosive, and he believed the use of gas was a tactic that would help to shorten the war – apparently the same calculation that was behind the gas attack in Douma by the Assad regime this month. In the bleakest irony of all, Haber’s related work on gases that could be deployed as pesticides would lay the groundwork for the creation of Zyklon B, the chemical used by the Nazis in the gas chambers of the Holocaust to kill other European Jews, including his own relatives.

Haber’s wife, Clara Immerwahr, however, saw her husband’s war work as an abomination. The first woman to be awarded a doctorate in chemistry at a German university, she denounced his research as a “sign of barbarity, corrupting the very discipline which ought to bring new insights into life”. Haber, in turn, is said to have regarded her opposition as a form of treason.

A few weeks after the first use of chlorine, and on the eve of Haber returning to the front to supervise another gas attack, Immerwahr fatally shot herself after an argument with her husband following a party to celebrate his “success”.

The story of Haber and Immerwahr is more than 100 years old. But its contemporary relevance is clear. As we have seen since evidence of the Douma attack emerged on 8 April, how we think about the use of poison gas – and about the moral requirement to respond to it – still matters.

Recently some have asked the following rhetorical question: why is it that we regard the apparent use of chemical weapons by the Assad regime (which has claimed relatively few lives overall) as more terrible than the crude pummelling by conventional arms which have resulted in hundreds of thousands of Syrian deaths?

The reality is that even before Haber began his lethal work, poison gas had been singled out for censure. In the Hague convention of 1899, a series of humanitarian principles were set out, which dealt with the treatment of prisoners and non-combatants and would later form the basis of the modern law of conflict. A key section was article 22, on the “right of belligerents to adopt means of injuring the enemy”. It declared that this right was “not unlimited”.

Importantly, its first prohibition concerned the employment of “poison or poisoned arms”. This built on an even earlier objection. In 1675, France and Germany had agreed to ban the use of poisoned bullets.

But it was not to be the desiccated language of a treaty that would give the proscription of chemical weapons – or the “accessory”, as allied troops during the first world war would euphemistically call their own gas – its real force, but reality described by witnesses such as the Canadian soldier, Hunter. For the generation that survived a conflict in which gas attacks accounted for an estimated 91,000 deaths, it was this viscerally recalled experience that reinforced a pre-existing but barely tested taboo.

That said, the Hague convention, not in that single article, but in its totality, played an important role. It put forward a vision, however much ignored in practice over the last century and more, that there should be hard legal limits on conflict, inspired by humanitarian concerns.

Gas was singled out, in the first instance, because it inspired a particular horror, in large part psychological. But the reason it has remained a special case is because of the way its prohibition has become emblematic of restrictions on warfare. We decided gas must not be used because of our horror of being gassed ourselves. As a result it became a taboo, any breach of which was viewed as an assault on the wider principles set out in the Hague convention, and later in the Geneva protocol of 1925.

The argument that relies on the idea that other weapons are equally deadly misses the point, which is that we have decided that this class of killing – like the wanton murder of civilians and shooting prisoners – is beyond the pale.

All of which means that even if you disagree with the military action taken by the UK, France and the US at the weekend, you cannot simply ignore the need to uphold the norm against chemical weapons use.

The alternative is to accept that the fitful advances in the laws of war – contradictory and permissive as they remain – are optional and reversible. Which, in the final analysis, was Haber’s position.

An outsider who felt he needed to prove his loyalty, Haber squandered a legacy that would have seen him hailed only as the father of modern fertilisers who, in his own words, “made bread from air”.

Instead, he is remembered as the heartless scientist who not only made gassing humans on the battlefield possible but also paved the way, both technically and in terms of creating a permissive moral space, for a far greater crime: the industrial gassing of his fellow European Jews in the Holocaust.

Haber was wrong about shortening the war. His use of gas precipitated its use by Germany’s foes three months later, ultimately entrenching the stalemate.

And today it is Clara Immerwahr, the woman who could not keep silent, that is celebrated for her vision and courage.